A police guard who was supposed to guard the hospital opened fire inside, according to city police spokesman Hashmatullah Stanekzai. The police guard shot himself but survived.

It's unknown if the Americans live in the Susquehanna Valley. One of the victims was a CURE International doctor named Dr. Jerry Umanos.

According to a statement from CURE International, Umanos served as a pediatrician at the hospital for more than seven years.

The U.S. Embassy in Kabul confirmed the killings and issued the following statement:

"The U.S. Embassy condemns the deplorable act of violence at the CURE International Hospital of Kabul today, which resulted in the deaths of three American citizens. Those killed in the attack were humanitarian workers dedicated to improving the lives of Afghans by providing a full range of medical services including reconstructive surgery to children with physical disabilities and pre- and post-natal care for mothers. This act of terror has deprived the citizens of Afghanistan of valuable medical expertise.

We offer our condolences and sympathy to the families of those who lost their lives or were injured while selflessly working towards a better future for the people of Afghanistan."

CURE International was established in 2005 in Lemoyne and specializes in maternal and child health. It treats 37,000 patients every year.

Afghanistan has seen a spate of deadly attacks against foreigners in recent weeks.

On April 4, two Associated Press journalists were shot in Afghanistan's eastern Khost province. Award-winning German photographer Anja Niedringhaus was killed, and Canadian reporter Kathy Gannon was injured.

And last month, five militants set off a deadly car bomb and then stormed a guest house used by foreigners in Kabul, the Afghan interior ministry said.

The militants held several foreigners hostage, including three Americans, a Malaysian and a person from an unspecified African country.

One girl was killed in the bombing. By the end of the hostage ordeal, One of the militants was shot and killed, and the other four blew themselves up.

The Taliban claimed responsibility for the March attack. Taliban spokesman Zabiullah Mujahid said militants targeted a location that foreigners used as a church and for converting Afghans to Christianity.